Fixed my iCloud issues:
Delete the Accounts folder here:
Then restart. Delete any extra iCloud accounts un Internet Accounts in System Preferences.
If you don't have any extra iCloud accounts in the list, you should be able to go ahead and check the box next to all of the iCloud apps and be fine.
Hope this helps.
I also had major iCloud issues.
In my case though, I had a phantom @gmail
account as well as my @icloud
account. My fix was as follows:
1) Log out of iCloud Keychain on all other devices (iPhone, iPad, other Macs etc.)
2) Delete account file (above)
4) Delete all iCloud accounts in System Preferences > Internet Accounts (click 'Later' on all error pop-up messages, do NOT log in to iCloud or provide your password)
5) Restart (probably not needed, but as a precaution)
6) Log into iCloud as normal